1. Technical Field
The present invention relates to ophthalmology. Particular embodiments relate to multi-vision ophthalmic lenses such as bifocals.
2. Discussion of Art
Throughout a human lifespan, ocular muscles continually exert focusing forces on the lens and ball of the human eye. Over time, the eye ball and lens deform permanently. This deformation results in a “multi-vision” effect wherein a person has difficulty focusing on both near and distant objects. Accordingly, two main categories of multi-vision lens have been developed: bifocals or progressives.
Bifocals have two distinct regions of differing optical power, typically a lower region for near vision and an upper region for distant vision. Wearers of bifocals are familiar with the disconcerting “jump” and “bend” that can occur at the defined line between the near and distant focal regions.
On the other hand, conventional progressives have a “progressive corridor” of gradually varying optical power that extends along an umbilical coil that joins a distant vision region of a first optical power with a near vision region of a second optical power. Surrounding the progressive corridor and the near and distant vision regions, progressives have a blended region of generally continually varying focal strength. The continual variance of focal strength eliminates jump, but often introduces astigmatism along with other visual defects.
Despite many advances in the field of multi-vision lenses, lens designers continue to face a trade off between undesirable image jump at a bifocal line, or undesirable image distortions outside a narrow progressive corridor. Thus, it remains desirable to provide a lens that avoids both bifocal jump and the astigmatism associated with standard progressive lens designs.